Sea maps and nautical charts represent an unbroken thread between all sailors throughout history. Though GPS and other electronics are becoming more and more prevalent, even they are just new version of the consistently refined and improved navigation methods that seafarers have depended on for centuries. In recent weeks we've covered numerous aspects of sailing, but today we look at one central to a maritime bookstore like ours: how to plot on a nautical map.

How to plot nautical chart

What does 'Plotting a Course' mean?

Navigational charts are on the oldest and most fundamental tools used by professional, commercial and leisurely sailors. Their modern incarnations provide a graphic image of marine environments across the globe, including the general arrangement of the sea bed, water depths, currents, locations of dangers/aids to navigation in not just the oceans but inland and coastal waters as well.

Each chart is based around a set of geographic co-ordinates that can be used to describe a specific location on the water, with an extensive set of chart symbols helping to realize the location. Using the latitude-longitude system, a navigator can measure their geographical position using the scales on the outer border of a chart, and by drawing a straight line between two points on the map, one can easily work out the compass direction for their desired route, as well as the distance (as one minute of latitude on a map corresponds to one nautical mile) needed to work out a course between both points.

What Plotting Tools Do You Need?

An Applicable Nautical Chart

Plymouth Chartadmiralty chart

Nautical charts can cover an entire country at large and its surrounding seas, to close-up views of single channels, port approaches, estuaries - you name it. It's paramount that you find the sea map that best covers all the areas you'll be sailing in, and whether you're opting for Admiralty or Imray charts you can do this with Seachest's Route Planner, and our blog post on how to find a nautical chart by content.

A Parallel Plotter 

A mounted ruler that lets you plot courses and parallel lines without the space required of traditional parallel rulers and/or triangle protractors. The Breton-Type Portland plotter is one of the most popular of these chart instruments, widely used by RYA practical and shore-based instructors, and engineered for both strength and accuracy.



To aid in measuring distance, dividers are a standard navigational tool that haven't changed much over the last 400 years. Different models provide varying degrees of convenience; the 'singlehaded' dividers for example have 'bow' pattern arms that cross over so they can be easily adjusted with one hand.

No. 2 Pencil/Chart Corrector Pen

Nautical Chart Corrector Pen

As familiar as a 2B pencil may seem to you, to seafarers they're the ideal scribing tool for chartwork; being soft and therefore kind to the chart, and yet they can be so hard to find in shops. Likewise, Chart Correction Pens are ideal for marking changes and adjustments, using chemically formulated ink that doesn't bleed once it dries.

How to Plot Your Course

  1. With your parallel plotter, mark out a straight line between your point of departure to the end point of your course. If you have one or more turns to make in your course, draw as many lines as necessary.
  2. Lay one side of the parallel ruler across the drawn line, and 'transfer the angle' by straightening and closing the parrell rule, each time holding and moving the alternate rules (known as 'walking' the rule) till the edge intersects the crossed lines at the centre of the compass rose on the chart.
  3. Using your dividers, or a Speed Time Distance Calculator (if you have one), and the distance scale on the top of bottom of the chart, determine the distance of each course in nautical miles. Do this for every line you've drawn, and write the calculated distance below the line (written as e.g. 1.9 NM).
  4. You can calculate the time it will take to sail each course by determining your vessel's speed in knots (based on average cruising speed and current weather conditions). Write this on your course line as well as e.g. 13 KTS.
  5. Multiply the distance of the course by 60, then divide that number by your predetermined speed in knots to calculate the amount of time it will take. Do this and write it on the bottom of your course line in minutes and seconds.

It's important that you start a stopwatch when your begin to run your course, stopping and starting it when starting another course whilst your turn and steady the boat, that you plot the course with heed to buoys and other aids to safe navigation and that you always deviate from a plotted course where necessary e.g. to avoid collision or dangerous conditions.

If you have any essential hints and tips for plotting on a nautical maps for any first time seafarers, you can share them in the comments below, or ship 'em out to us on our Facebook page, through Twitter and also via Google+.



Post By Graham